1. Field of the Invention
The present invention relates generally to a multiple-input multiple-output (MIMO) system, and more particularly to a method for feeding back antenna shuffling information in a MIMO system using a layered space-time block coding technique or double space-time transmit diversity (DSTTD) technique.
2. Description of the Related Art
A large amount of research is being conducted in the field of wireless mobile communication systems for providing multimedia service in which high-quality, high-speed, and high-capacity data can be transmitted. Wireless channel environments of wireless mobile communication systems are different from wired channel environments, and receive, in practice, a distorted transmitted signal due to various factors such as multipath interference, shadowing, radio wave attenuation, and time-varying noise and interference. Fading due to the multipath interference is closely associated with a reflecting object or user, and the mobility of a user terminal. When the fading due to the multipath interference occurs, a signal composed of a real transmitted signal mixed with an interference signal is received. The received signal is seriously distorted from the originally transmitted signal, such that overall performance of the mobile communication system is degraded. Because the effect of fading can distort the amplitude and the phase of the received signal, it is an important factor that interferes with high-speed data communications in the wireless channel environment. A large amount of research is currently being conducted to prevent the fading effect. To transmit data at high speed, the mobile communication system must minimize the loss and any user-by-user interference according to the characteristics of a mobile communication channel. To compensate the loss and overcome the interference, many multiple-input multiple-output (MIMO) techniques have been proposed.
The MIMO technology can be divided into various categories of techniques according to the data transmission method used and the presence of channel information feedback.
The MIMO technology is divided into categories such as spatial multiplexing (SM) and spatial diversity (SD) techniques according to the data transmission method. The SM technique simultaneously transmits different pieces of data using multiple antennas at a transmitting terminal and a receiving terminal, thereby transmitting data at high speed without increasing the system bandwidth. The SD technique transmits identical data by means of multiple transmit (Tx) antennas or transmit symbol times, thereby obtaining transmission diversity.
The MIMO technology is further divided into categories such as open and closed loop techniques according to the presence of channel information feedback in the receiving terminal.
The closed loop technique is a singular value decomposition (SVD) technique. The SVD technique can theoretically obtain optimal performance, but has a disadvantage in that the number of computations increases because the receiving terminal must feed back all of the channel values to the transmitting terminal.
The open loop technique is divided into categories such as space-time block coding (STBC), Bell Labs layered space-time (BLAST) and layered STBC (L-STBC) techniques according to the data transmission method. The STBC technique has been proposed to support a Tx antenna diversity function. However, when the number of Tx antennas increases, diversity gain decreases. According to the BLAST technique, a data rate is high, but performance is degraded because a diversity gain is absent. In the BLAST technique, the number of receive (Rx) antennas must be greater than or equal to the number of Tx antennas. To overcome the disadvantages of the STBC and BLAST techniques, the L-STBC technique has been proposed. The L-STBC technique is a combination of both the STBC and BLAST techniques, and improves the diversity gain and the data rate as compared with both the STBC and BLAST techniques, and obtains both of the diversity gain and multiplexing gain.
The structure of a conventional MIMO communication system will be described with reference to FIGS. 1 and 2.
FIG. 1 relates to an article by: N. Prasad and M. Varanasi, entitled “Optimum Efficiently Decodable Layered Space-Time Block Codes”, Signals, Systems and Computers, 2001. Conference Record of the Thirty-Fifth Asilomar Conference, vol. 1, pp. 227-231, 2001. FIG. 2 relates to an article by: E. N. Onggosanusi, A. G. Dabak, and T. M. Schmidl, entitled “High Rate Space-Time Block Coded Scheme: Performance and Improvement in Correlated Fading Channels”, WCNC 2002—IEEE Wireless Communications and Networking Conference, vol. 3, no. 1, March 2002, pp. 161-166.
FIG. 1 is a block diagram illustrating a conventional L-STBC system.
Referring to FIG. 1, a transmission and reception system using the L-STBC technique has an open loop structure. That is, the receiving terminal does not feed back channel information to the transmitting terminal, and the transmitting terminal does not identify channel information. Accordingly, the transmitting terminal does not perform adaptive modulation according to a channel state. Because the receiving terminal must sequentially eliminate noise components from the signals transmitted from the transmitting terminal, complexity increases due to frequent iteration.
FIG. 2 is a block diagram illustrating a conventional system using double space-time transmit diversity (DSTTD) technique for feeding back a weighting matrix.
Referring to FIG. 2, the conventional DSTTD system has a closed loop structure for feeding back a weighting matrix from the receiving terminal to the transmitting terminal. The transmitting terminal receiving the weighting matrix multiplies an STBC signal by a weighting value and transmits the STBC signal multiplied by the weighting value to the receiving terminal, such that STBC diversity performance can be obtained from a correlated channel. However, the number of computations required to obtain an optimal weighting matrix is very large. The calculation of the weighting matrix imposes a heavy burden on the receiving terminal in FDD system. Accordingly, the receiving terminal just intermittently feed back information of the weighting matrix.
However, the amount of information of the weighting matrix fed back from the receiving terminal is also a heavy burden. Accordingly, the DSTTD system replaces the weighting matrix with a permutation matrix and uses antenna shuffling in an assumed independent, identically distributed (i. i. d.) channel environment. However, a weighting matrix optimal to a correlated channel does not have an equation to approximate the closed loop structure. The weighting matrix fed back by the receiving terminal in FIG. 2, which is initially set up at the transmitting terminal, is a result value produced by many simulations.